Today's market for metal beverage cans is extremely price competitive, which necessitates making the cans from the least amount of metal possible while still providing the necessary structural integrity. By using state of the art manufacturing techniques it is now possible to manufacture a 12-ounce aluminum can having a thin side wall, e.g. , about 0.0040-0.0045 inch thick, with the side wall increasing in thickness at its upper end to about 0.0070-0.0075 inch to permit the forming of a can neck without collapsing or wrinkling the side wall. New necking processes are expected to yield metal and cost reduction in the neck region of the can. An example of such a process is the spin flow process disclosed in U.S. patent application Ser. No. 07/929,932, filed Aug. 14, 1992, and issued to the present assignee, Reynolds Metals Company. U.S. Pat. No. 4,781,047 issued to Ball Corporation also pertains to a spin flow process.
Notwithstanding the technological advances which have resulted in metal savings in the neck region of the can, the can bottom continues to be manufactured with a thickness of about 0.012 inch (which actually varies somewhat across different parts of the bottom due to drawing action), which means that about one-third of the weight of the metal in the can must be in the bottom, to provide the necessary structural integrity. The can bottom must be able to sustain a column load of approximately 250 pounds during a spin flow necking process and 300 pounds during a die necking process. Later, it must sustain a column load of about 135 pounds when a can end is double-seamed on the can body after it has been filled with product. Another design criterion is a drop test for shock loads, in which the filled and seamed can must be able to resist a drop of about five inches without bottom reversal or increase in can height. In addition, a can filled with a carbonated beverage must be able to contain an internal pressure of about 40-100 psi.
To meet these requirements, conventional industry practice is to form the relatively thick bottom to have a profile with a concave or hollow central region. The bottom is formed into its final, inwardly domed shape between a hollow die engaging the internal surface of the bottom from the interior of the can and a punch engaging the external surface of the bottom. Cooperation of the punch and die creates a bottom having an inner wall at the outside of the concave region, an outer wall connected to the can side wall, and a rest radius connecting these two inner and outer walls.
The resistance of the inwardly domed portion to outward bulging under internal pressure is greatly influenced by the size of the rest radius. The smaller the rest radius, generally the higher the internal pressure resistance of the can. Too large a radius will reduce this pressure to an unacceptable level. However, this conventional forming process works best if the rest radius is large, because during the process the sheet metal is pulled radially inward into the hollow region and, as viewed in profile, snakes around the radius on the punch and die. Too small a radius will create a fracture or thickness reduction. Thus, these two competing factors require compromise. Although advances are presently being made by the present assignee and others to reduce the rest radius of can bottoms to increase their bulge strength and thereby reduce their thickness, this approach inherently requires that the overall strength of the can bottom is dictated by mechanical features in the bottom.
U.S. Pat. No. 5,105,973, which issued Apr. 21, 1992 to Ball Corporation, contains a comprehensive discussion of inwardly domed can bottoms and the phenomena of dome reversal and roll-out (i.e., unrolling of inward profiles) caused by internal pressure, increases in overall can height resulting from this type of failure, and ways to strengthen inwardly domed can bottoms without unacceptably decreasing the internal volume of the can. See also U.S. Pat. Nos. 4,722,215 and 4,885,924 issued to Metal Box p.l.c., which concern reforming inwardly domed can bottoms in an additional operation, and U.S. Pat. Nos. 4,177,746 and 4,222,494 issued to Reynolds Metals Company, the assignee hereof.
It is an object of the present invention to reduce the thickness of the metal in a can bottom without significantly affecting the structural integrity of the can.
Another object is to reduce the metal in the can bottom to a thickness of approximately 0.0070 inch to thereby reduce its weight by approximately 30% while still enabling the can to satisfy design requirements.
Yet a further object of the invention is to provide a can bottom formed without inwardly curved mechanical features.
A further object is to provide a can bottom wherein the tensile strength of the metal provides sufficient strength to satisfy the design requirements.